Method Of Production Of High Purity Silver Particles

ABSTRACT

A method for synthesizing high purity silver particles and colloids without requiring the addition of either surfactants or reducing agents thereto, or requiring only a minimal amount thereof. The synthesizing process comprises: (i) a silver oxalate synthesizing process; (ii) a process of dispersing silver oxalate into an appropriate carrier; and {iii) a process of heating said silver oxalate dispersed into said carrier at a temperature of at least 100° C. Silver particles and colloids of various form factor and size may be synthesized depending upon the reaction conditions, the carrier, and the type of surfactant.

This application claims the benefit of U.S. Provisional Application No.60/618,876, filed Oct. 14, 2004 and entitled “Method for Production ofFine Silver Particles,” which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a method of forming silver particles bydispersing silver oxalate into an appropriate carrier and then applyingheat at a temperature of 100° C. or higher to decompose the silveroxalate.

BACKGROUND ART

A number of methods have been developed to synthesize silver particlesincluding, but not limited to, chemical reduction, photochemical,sonochemical and gas evaporation methods. Of these methods, the chemicalreduction method is widely used due to the ease of production. However,silver powder produced using the chemical reduction method can becontaminated by the reducing agent, the surfactant and impurity ionsused during the reaction process, which can serve as a limiting factorin the field of electronics requiring high conductivity or in the fieldof bacteria resistance requiring high purity.

In order to resolve these problems, it is desirable to have a method ofproducing high purity silver powder and silver colloids that does notrequire either surfactants or reducing agents, or only a minimal amountof a surfactant.

DISCLOSURE OF THE INVENTION

The object of the present invention is to synthesize high purity silverparticles and colloids in a process that does not require eithersurfactants or reducing agents, or only a minimal amount of asurfactant. In the present invention, this object is achieved bydispersing silver oxalates into an appropriate carrier and thenthermally decomposing the silver oxalates at a temperature of 100° C. orhigher to synthesize high purity silver particles and colloids.

The process of synthesizing silver particles and colloids by the methodof the present invention comprises: (i) a silver oxalate synthesizingprocess; (ii) a process of dispersing silver oxalates into anappropriate carrier, for example, water, alcohol or the like, includinga combination of more than one carrier; and (iii) a process of heatingsaid silver oxalates dispersed into said carrier at a temperature of100° C. or higher under a pressure greater than atmospheric pressure.

These and other features, objects and advantages of the presentinvention will become better understood from a consideration of thefollowing detailed description of the preferred embodiments and appendedclaims in conjunction with the drawings as described following.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a microphotograph of silver particles obtained under theconditions described in Example 1.

FIG. 2 is a microphotograph of silver particles obtained under theconditions described in Example 2.

FIG. 3 is a microphotograph of silver particles obtained under theconditions described in Example 3.

FIG. 4 is a microphotograph of silver particles obtained under theconditions described in Example 4.

FIG. 5 is a microphotograph of silver particles obtained under theconditions described in Example 5.

FIG. 6 is a microphotograph of silver particles obtained under theconditions described in Example 6.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1-6, the preferred embodiments of the presentinvention may be described as follows.

In the present invention, a method for the production of silverparticles and colloids comprises three processes as follows: (i) asilver oxalate (Ag₂C₂o₄) synthesizing process; (ii) a process ofdispersing silver oxalate into an appropriate carrier, for example,water, alcohol or the like, including a combination of more than onecarrier; and (iii) a process of heating said silver oxalate dispersedinto said carrier at a temperature of 100° C. or higher under a pressuregreater than atmospheric pressure to form silver particle or colloidsfrom the decomposition of the silver oxalate.

A first solution of a water soluble silver compound and a secondsolution of an oxalate compound are mixed together to precipitate silveroxalates. The silver compound may be AgNO₃. The oxalate compound may besodium oxalate or oxalic acid. The present invention is not, however,limited to these specific compounds but may include any two solutions ofcompounds that form silver oxalates upon mixing. After water cleaningprocesses, preferably two or more rounds of water cleaning processes,are performed to remove impure ions from the precipitated silveroxalate, the silver oxalate is used as the starting material forsynthesizing silver powder or colloids.

The synthesized silver oxalate is dispersed into an appropriate carrier.The silver oxalate is not dissolved to any substantial extent in thecarrier, but is dispersed as solid particles by using ultrasonictreatment. The appropriate carrier may include all types of carrierswhich can disperse silver oxalate to effectively deliver heat. Thecarrier is selected to have properties that allow it to behave similarlyto a surfactant so as to prevent agglomeration of the silver particlesformed from the thermal decomposition of the silver oxalate. Forexample, alcohols consist of alkyl and hydroxyl groups. Generally, alkylgroups have hydrophobic properties and hydroxyl groups have hydrophilicproperties. Organic materials having both hydrophobic and hydrophilicproperties can play a role as a surfactant. However, organic materialshaving higher carbon numbers tend to be dominantly hydrophobic and maytherefore tend to lose the ability to act as a surfactant in the processof the present invention. Generally, organic materials having highernumbers of carbon atoms have superior surfactant properties. However, inthe present invention, organic materials with a higher number of carbonatoms is observed to agglomerate silver particles. Furthermore, organicmaterials with a higher number of carbon atoms do not mix well withwater. Therefore, the present invention is limited to methyl, ethyl andpropyl alcohols, which have a low number of carbon atoms. Water is alsoeffective in the practice of the present invention. The appropriatecarrier may therefore consist of ethyl alcohol, methyl alcohol, propylalcohol, water or a combination of more than one of the preceding.

The carriers selected for the practice of the present invention all havelow boiling points: water (100° C.), methyl alcohol (64.65° C.), ethylalcohol (78.3° C.), and propyl alcohol (82° C.). Accordingly, when thecarrier with the dispersed silver oxalate is heated in a container at orabove 100° C., the pressure is always above atmospheric pressure.Typical reaction pressures are about 1.86*10⁵ N/m² when using water asthe carrier and about 5.31*10⁵ N/m² when using ethyl alcohol as thecarrier. During thermal decomposition of silver oxalate, the silveroxalate (Ag₂C₂O₄) decomposes into silver (Ag) and carbon dioxide (CO₂)according to the formula Ag₂C₂O₄=2Ag+2CO₂. The carbon dioxide gasevolved during the thermal decomposition of the silver oxalate and thecarrier vapor may be evacuated as necessary but pressure drops of lessthan about 6.89*10⁴ N/m² do not affect the quality of the silverparticles.

The dispersed silver oxalate in the carrier is placed into a closedreactor to heat the dispersed silver oxalate and carrier up to at least100° C. to synthesize silver powder or colloids of various form factors.

This method may optionally use surfactants in order to preventcoagulation or agglomeration of the silver particles. Surfactants may beadded to the water soluble silver or oxalate solutions used to producesilver oxalate, or may be added after the silver oxalate is produced bymixing the two solutions. Surfactants used in this method may includeanionic surfactants, cationic surfactants, amphoteric surfactants,nonionic surfactants, fluorochemical surfactants, and polymerizablesurfactants, or combinations of the preceding, which may be added to aidin forming silver particles and to break down silver plates or preventsilver plates from coagulation. Surfactants suitable for use in thepresent invention include PVP (polyvinyl pyrrolidone) and gelatine.

Irrespective of the amount of surfactant added, silver particles orcolloids can be obtained by the method of the present invention,however, it is desirable to limit the amount of surfactant to no morethan 80% of the weight of the silver. For example, if 10 grams of silveris placed into the reactor, the weight of the surfactant, such as PVP orgelatin, should be no more than 8 grams.

EXAMPLE 1

After 2.8 grams of silver oxalate was placed into 300 cc of distilledwater, ten minutes of ultrasonic treatment was performed to disperse theparticles. The dispersed silver oxalate was reacted for 15 minutes at130° C. to obtain a solution containing silver particles as shown inFIG. 1.

EXAMPLE 2

After 28 grams of silver oxalate was placed into 1000 cc of ethylalcohol, ten minutes of ultrasonic treatment was performed to dispersethe particles. The dispersed silver oxalate was reacted for 15 minutesat 134° C. to obtain silver powder as shown in FIG. 2.

EXAMPLE 3

After 70 mg of silver oxalate was placed into 1000 cc of ethyl alcohol,ten minutes of ultrasonic treatment was performed to disperse theparticles. The dispersed particles were reacted for 25 minutes at 135°C. to obtain nano-sized silver particles as shown in FIG. 3.

EXAMPLE 4

After 4.2 grams of silver oxalate was placed into a mixed solution ofwater (vol. 50%) and ethyl alcohol (vol. 50%), the solution was reactedfor 15 minutes at 130° C. to synthesize 0.5 μm silver particles as shownin FIG. 4.

EXAMPLE 5

30 wt % of PVP (polyvinyl pyrrolidone) was placed into 4.2 grams ofsilver oxalate in 1 Liter of water and ultrasonic treatment wasperformed to disperse the particles thereof. The dispersed particleswere reacted for 20 minutes at 135° C. to synthesize silver particles of0.5 μm or smaller in size as shown in FIG. 5.

EXAMPLE 6

10 grams of gelatin was placed into 28 grams of silver oxalate in 1Liter of water and ultrasonic treatment was performed to disperse theparticles thereof. The dispersed particles were reacted for 15 minutesat 135° C. to synthesize silver particles of 50 nm or smaller in size asshown in FIG. 6.

INDUSTRIAL APPLICABILITY

Due to its inherent characteristics of high conductivity and bacteriaresistance, silver particles are widely used in the electronics industryas well as in other industries requiring bacteria resistance.

The present invention has been described with reference to certainpreferred and alternative embodiments that are intended to be exemplaryonly and not limiting to the full scope of the present invention as setforth in the appended claims.

1. A method for production of silver particles, comprising the steps of:(a) dispersing solid silver oxalate particles in a carrier; and (b)heating said dispersed silver oxalate to a temperature of at least 100°C. under a pressure that is greater than atmospheric pressure todecompose said silver oxalate into silver particles.
 2. The method ofclaim 1, further comprising the step before step (a) of producing saidsilver oxalate by mixing a first solution of a silver compound and asecond solution of an oxalate compound to form silver oxalate.
 3. Themethod of claims 1 or 2, further comprising the step of adding asurfactant to said dispersed silver oxalate before the heating of step(b).
 4. The method of claim 3 wherein said surfactant is selected fromthe group consisting of anionic surfactants, cationic surfactants,amphoteric surfactants, nonionic surfactants, fluorochemicalsurfactants, polymerizable surfactants, and any combination of thepreceding surfactants.
 5. The method of claim 2, further comprising thestep of adding a surfactant to said first solution before said mixingstep.
 6. The method of claim 5 wherein said surfactant is selected fromthe group consisting of anionic surfactants, cationic surfactants,amphoteric surfactants, nonionic surfactants, fluOrochemicalsurfactants, polymerizable surfactants, and any combination of thepreceeding surfactants.
 7. The method of claim 2 further comprising thestep of adding a surfactant to said second solution before said mixingstep.
 8. The method of claim 7 wherein said surfactant is selected fromthe group consisting of anionic surfactants, cationic surfactants,amphoteric surfactants, nonionic surfactants, fluorochemicalsurfactants, polymerizable surfactants, and any combination of thepreceeding surfactants.
 9. The method of claims 1 or 2, wherein thedispersing of step (a) comprises adding silver oxalate to said carrierand subjecting the mixture of silver oxalate and carrier to ultrasonictreatment.
 10. The method of claims 1 or 2, wherein said carrier isselected from the group consisting of water, methyl alcohol, ethylalcohol, propyl alcohol, and a mixture of any of the preceding.
 11. Themethod of claim 2, wherein said silver compound is AgNO₃.
 12. The methodof claim 2, wherein said oxalate compound is selected from the groupconsisting of sodium oxalate and oxalic acid.
 13. The method of claim 2,further comprising the step following the production of silver oxalateof washing said silver oxalate with water to remove impurities.
 14. Themethod of claim 4, wherein said surfactant is selected from the groupconsisting of PVP (polyvinyl pyrrolidone) and gelatin.
 15. The method ofclaim 6, wherein said surfactant is selected from the group consistingof PVP (polyvinyl pyrrolidone) and gelatin.
 16. The method of claim 8,wherein said surfactant is selected from the group consisting of PVP(polyvinyl pyrrolidone) and gelatin.
 17. The method of claim 3, whereinsaid surfactant is no greater than 80% by weight of the silver in step(b).
 18. The method of claim 5, wherein said surfactant is no greaterthan 80% by weight of the silver in step (b).
 19. The method of claim 7,wherein said surfactant is no greater than 80% by weight of the silverin step (b).